Add Dedent Common Whitespace From Multiline String as a Python TIL

python/dedent-common-whitespace-from-multiline-string.md

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+# Dedent Common Whitespace From Multiline String
+
+I'm adding tests to cover some custom logic in the _usage_ messages that are
+produced by Click [in a CLI I'm building](https://github.com/jbranchaud/py-vmt).
+The usage output is a mutliline string formatted with some indentation. On top
+of that, I'm putting it in a Python test file with some additional syntactical
+indentation.
+
+If this was Ruby, I would reach for the `<<~` heredoc syntax. The closest
+equivalent to that in Python is
+[`textwrap.dedent`](https://docs.python.org/3/library/textwrap.html#textwrap.dedent).
+
+> Remove any common leading whitespace from every line in text.
+> 
+> This can be used to make triple-quoted strings line up with the left edge of
+> the display, while still presenting them in the source code in indented form.
+
+```python
+import textwrap
+
+def test_base_help_message():
+    runner = BetterCliRunner()
+
+    help_result = runner.invoke(cli, ["--help"])
+    output = textwrap.dedent("""\
+    Usage: vmt [OPTIONS] COMMAND [ARGS]...
+
+    Options:
+      -v, --verbose  See extra output when running commands
+      --help         Show this message and exit.
+
+    Commands:
+      cancel
+      log
+      start
+      status
+      stop
+    """)
+    assert output in help_result.output
+```
+
+Before adding in `textwrap.dedent`, the test was failing because of bits of
+extra whitespace that wasn't matching up with the actual CLI output.
+
+Note: the leading `\` after the opening `"""` is needed to remove the leading
+newline that would otherwise be included.